Novel alkoxyamine linker to load ketones for solid-phase synthesis: Application of the synthesis of 1,4-benzodiazepine-2-ones

Full text of DOI:10.1021/cc9001795

J. Comb. Chem. 2010, 12, 311–314
311
of benzodiazepine.6 After Boc-protected R-amino acids 14
are condensed with 13, the nitrogen on the aromatic ring is
functionalized with alkylating agents. Finally, removal of the
Novel Alkoxyamine Linker to Load Ketones
for Solid-Phase Synthesis: Application of the
Synthesis of 1,4-Benzodiazepine-2-ones
Scheme 1. Solid-Phase Synthesis of Heterocyclic
Compounds by Using a Traceless Alkoxyaniline Linker 1
Kimihito Matsushita, Chieko Okamoto,
Mayumi Yoshimoto, Kenichi Harada, Miwa Kubo,
Yoshiyasu Fukuyama, and Hideaki Hioki*
Faculty of Pharmaceutical Sciences, Tokushima Bunri
UniVersity, Yamashiro-cho, Tokushima 770-8514, Japan.
ReceiVed NoVember 21, 2009
Selection of an adequate linker is important for the solid-
phase organic synthesis. Linkers should be easy to load
scaffolds onto the solid support, and must be not only stable
during the reactions but also cleavable without product
damage at the final stage. Much effort has been made to
design and synthesize various linkers suitable for loading
the scaffolds, as well as for giving desired products.1 We
have previously developed the new alkoxyaniline linker 1,
which is applicable to the solid-phase synthesis of hetero-
cyclic compounds such as 2-substituted benz-fused azoles
6, quinazolines 7, and quinazolinones 8.2 The linker 1 is
characterized as a traceless linker, in which no functional
group of the target molecules is necessary to attach to a solid
support.3 The reaction sequence to synthesize these hetero-
cyclic compounds involved loading of the aromatic aldehydes
on the solid support by an imine synthesis and oxidative
release of resin-bound imines with various 2-substituted
anilines. The releasing step includes the equilibrium of the
imine-exchange process between 2 and 4 as shown in
Scheme 1. Although the stability of compounds 2 and 4 is
comparable, the equilibrium can be shifted to the product
side by coupling irreversible oxidation process.
Scheme 2. Strategy for Releasing Product by Intramolecular
Imine Exchange Reaction
Scheme 3. Synthetic Plan for the Solid-Phase Synthesis of
1,4-Benzodiazepine-2-ones
In this study, we have focused on an intramolecular
imine exchange reaction to expand the utility of our
strategy. Releasing product from the solid support by the
intramolecular nucleophilic attack of amine group is
entropically favorable as a consequence of less adverse
∆S when forming seven-membered ring system. Thus the
equilibrium is expected to shift to product side in this
process4 (Scheme 2).
Solid-phase synthesis of 1,4-benzodiazepine-2-ones his-
torically set the stage for the combinatorial synthesis of
various small molecules on a solid support.5 They are
classified as privileged structure because of a wide range of
their pharmaceutical activities such as hypnosis and anxi-
olytic actions.6 Thus we have decided to synthesize 1,4-
benzodiazepine-2-ones 18 to demonstrate the validity of our
strategy outlined in Scheme 2. According to our synthetic
plan as shown in Scheme 3, 2-aminobenzophenones 12 are
needed to load on a solid support because the aromatic group
at the C-5 position in 18 is essential for the biological activity
* To whom correspondence should be addressed. Phone: +81 88 602
8437. Fax: +81 88 655 3051. E-mail: hioki@ph.bunri-u.ac.jp.
10.1021/cc9001795  2010 American Chemical Society
Published on Web 03/05/2010

312 Journal of Combinatorial Chemistry, 2010 Vol. 12, No. 3
Reports
Table 1. Benzophenone Hydrazones and Oximes Formation by
Using Various Hydrazines and Alkoxyamines
Scheme 4. Synthesis of 1,4-Benzodiazepine-2-one 27 in
Solutiona
entry
X
N
R1
R2
time (h)
yield (%)
1a
2a
3b
4a
5a
6a
7c
Me
Me
Me
H
H
H
H
48
48
20
20
48
40
40
0
0
4-methoxyphenyl
Ph
2,4-dinitrophenyl
C2H5OCO
19
42
77
13
98
H
H
C2H5NHCO
8a b
O
Ph
Me
48
20
0
91
,
9b
a TsOH was added as an acid catalyst.
b 19 was used as
hydrochloride salt. c 19 was used as trifluoroacetate salt.
Boc group under acidic conditions, together with a subse-
quent product release by intramolecular imine exchange
reaction, would give the desired benzodiazepines 18.
Since aromatic ketones were not easily loaded on the solid
support with the original alkoxyaniline linker 1 by simply
heating under the acidic conditions because of their low
reactivity,7 we first explored a suitable linker for loading
benzophenones on a solid support. However, the previously
known linkers to load ketones are quite limited8 in contrast
to those for amines, carboxylic acids, alcohols, and alde-
hydes.1 Ketohydrazones and ketoximes are known to be quite
stable, and some of them are simply obtained by heating
under the neutral or mild acidic conditions.9 Therefore,
reactions of various hydrazines 19 (X ) N) and alkoxyamines
19 (X ) O) with benzophenone were examined in solution
to afford the corresponding ketohydrazones 21 (X ) N) and
ketoximes 21 (X ) O). Results are summarized in Table 1.
Treatment of benzophenone with N-alkyl hydrazines did
not give the hydrazones10 (entries 1 and 2). In the case of
N-aromatic hydrazines, the desired hydrazones were obtained.
Their yields highly depended on the substituents on the
aromatic nuclei (entries 3-5). The use of a hydrazine with
electron-withdrawing group on the aromatic ring gave higher
yield (entry 5). The reaction with ethyl carbazate gave the
corresponding hydrazone in only 13% yield (entry 6). On
the other hand, the reaction with semicarbazide gave the
hydrazone in very good yield (entry 7). In a series of oxime
formation, the use of O-phenylhydroxylamine was not
effective10 but that of O-methylhydroxylamine gave the
corresponding oxime in 91% yield (entries 8 and 9).
Although the semicarbazide gave the best yield in this
screening (entry 7), we chose O-alkylhydroxylamine as an
appropriate linker to load benzophenones because the hy-
drazone derived form semicarbazide has highly acidic N-H
proton that may cause side reaction, such as N-alkylation at
this position in the course of further transformation.11
1,4-Benzodiazepine-2-ones 27 were synthesized in solution
to optimize the reaction conditions in each step (Scheme 4).
Benzylamine 22 was used as a solution model for aminom-
ethylated polystyrene resin. Boc-protected aminooxyhexanoic
acid12 was condensed with 22 to afford 23. After removal
a Reagents and conditions: (a) 6-(N-tert-butyloxycarbonylaminooxy)hex-
anoic acid, EDC, DMAP, DCM, rt, 3 h; (b) 15% TFA in DCM, rt, 30 min,
then 2-aminobenzophenone, EtOH, reflux, 38 h; (c) Boc-Gly-OH or Boc-
Phe-OH, EDC, DMAP, DCM, rt, 4 h; (d) MeI, Cs2CO3, DMF, rt, 4 h; (e)
TFA-H2O (4:1), reflux, 24 h. b Enantiomeric excess was determined by
chiral HPLC on the CHIRALCEL OD-H.
of the Boc group, the resulting amine was treated with
2-aminobenzophenone in ethanol under reflux. The desired
oxime 24 was obtained in good yield as a 2:1 geometrical
isomer, which was used without separation for further
reactions. The oxime 24 was subjected to condensation with
Boc-protected glycine or phenylalanine. The amides 25a and
25b were prepared in good yields. Subsequent N-methylation
of 25 proceeded smoothly at rt to give 26a and 26b in
excellent yields. At this stage, two geometrical isomers were
separated. Finally, removal of the Boc group and subsequent
intramolecular oxime-imine exchange reaction to release the
final product were investigated. Desired benzodiazepines
were obtained in good yields regardless of the geometry of
the oxime bond and substitution at C-3. When R is a benzyl
group, 27b slightly lost their chirality at the C-3 position
(27b 98% ee and 94% ee for each geometrical isomer around
CdN linkage in 26b).
Next, the optimized reaction conditions described above
were applied to the solid-phase synthesis of 1,4-benzodiaz-
epine-2-one (Scheme 5). The aminooxyalkylated resin 29
was synthesized in two steps from aminomethylated Syn-
Phase-PS Lantern 28, which is polystyrene grafted surface,
as follows. Boc-protected aminooxyhexanoic acid was loaded
through amide linkage, and then the Boc group was removed
under acidic conditions to afford TFA salt 29, which was
subjected to further reaction without neutralization. Treatment
of 29 with 4 equiv of 2-aminobenzophenone in EtOH under
reflux gave the corresponding oxime. The amino group on
the aromatic ring was acylated with Boc-protected glycine
or phenylalanine.13 The resulting amides were subjected to
N-methylation under the same reaction conditions as in
solution-phase. Removal of the Boc group triggered benzo-

Reports
Journal of Combinatorial Chemistry, 2010 Vol. 12, No. 3 313
Table 3. Enantiopurity and Isolated Yield of 33 under Various
Acylation Conditions
Scheme 5. Solid-Phase Synthesis of 1,4-Benzodiazepine-
2-ones 27 by using SynPhase-PS Lanterna
entry
additive
enantiomeric excess (%)a
isolated yield (%)
1
2
3
DMAP
HOBt
HOAt
36
99
99
53
21
73
a Reagents and conditions: (a) 6-(N-tert-butyloxycarbonylaminooxy)hex-
anoic acid, DIC, HOBt, DCM, rt, 24 h then 50% TFA in DCM, rt, 1 h; (b)
2-aminobenzophenone, EtOH, reflux, 48 h; (c) Boc-Gly-OH or Boc-Phe-
OH, DIC, DMAP, DCM, rt, 24 h; (d) MeI, Cs2CO3, DMF, rt, 3 h; (e)
TFA-H2O (4:1), reflux, 24 h. b Enantiomeric excess was determined by
chiral HPLC on the CHIRALCEL OD-H.
a Enantiomeric excess was determined by chiral HPLC on the
CHIRALCEL OD-H.
Scheme 6. Solid-Phase Synthesis of 1,4-Benzodiazepine-
2-one 27b under the Optimized Conditionsa
Table 2. N-Methylation on Various Solid Supports
a Reagents and conditions: (a) 2-aminobenzophenone, EtOH, reflux, 72 h;
(b) Boc-Phe-OH, DIC, HOAt, DCM, rt, 48 h; (c) MeI, Cs2CO3, DMF, rt,
48 h; (d) 5% TFA, 1,2-dichloroethane, 80 °C, 18 h. b Enantiomeric excess
was determined by chiral HPLC on the CHIRALCEL OD-H.
isolated yield (%)
entry
resin
Lantern
Lantern
polystyrene
composition
cross-linking
27b
30b
1
2
3
4
polystyrene
polyacrylamide
polystyrene
20
29
21
21
5
0
0
Table 4. Synthesis of 1,4-Benzodiazepine-2-ones 35 using
Several R-Amino Acids As Building Blocks
macroporous
1% DVB
1% DVB
0.8
50
StratoSpheres polystyrene
5a StratoSpheres polystyrene
73b
a Cleavage with 5% TFA in 1,2-dichloroethane at 80 °C for 18 h.
b Enantiomeric excess was 45% ee, which was determined by chiral
HPLC on the CHIRALCEL OD-H.
diazepine formation along with releasing the final products.
However, the desired products 27a and 27b were generated
only in 7% and 8% yields, respectively, along with a large
amount of 30a and 30b. In addition, considerable racem-
izations of them (32% ee for 27b and 18% ee for 30b) were
caused during these reaction sequences. The results indicated
that the reaction conditions of these steps in solution were
unable to be applied to the solid-phase synthesis.
The reaction conditions of N-alkylation were first exam-
ined to improve the yields. After numerous trials, we finally
found that the N-alkylation highly depended on solid-support
as shown in Table 2. The N-methylation was completed by
using polystyrene resin cross-linked with 1% divinylbenzene
(entry 4). The reactivity of the N-alkylation may be related
to swelling capacity of the solid support. Its yield was
improved to 73% overall when cleavage from the resin
carried out with 5% TFA in 1,2-dichloroethane (entry 5).
However, considerable racemization was still observed
(45% ee).
The acylation of 32 was examined again to suppress this
racemization as shown in Table 3. The use of 1-hydroxy-
benzotriazole (HOBt),14 which is known as a racemization-
suppressing agent, drastically increased the enantiomeric
excess of 33 to 99%, but decreased chemical yield by 21%
(entry 2). Addition of HOAt,15 which is known to efficiently
speed up coupling process and reduce a loss of chiral
integrity, gave the desired product 33 in a high enantiomeric
excess and good yield (entry 3, 73% yield and 99% ee). Thus
the problems associated with N-alkylation and racemization
at the C-3 position in the solid-phase synthesis of 1,4-
benzodiazepine-2-one have been solved. Benzodiazepine 27b
was also synthesized in 66% yield without racemization
under the optimized conditions as shown in Scheme 6.16
Finally, several R-amino acids were applied as building
blocks at the acylation step to examine the scope of this
method. All four benzodiazepines 35 were obtained in
moderate to good yields (Table 4).
In conclusion, we have developed a traceless alkoxyamine
linker suitable for loading ketones, and it has proved to be
useful for the solid-phase synthesis of 1,4-benzodiazepine-
2-ones. The reaction sequences involve four steps: loading
ketones as oximes, condensation of R-amino acids,

314 Journal of Combinatorial Chemistry, 2010 Vol. 12, No. 3
Reports
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N-alkylation, and cleavage of the products from the resin
by intramolecular oxime-imine exchange reaction. The
compounds 27b and 33, which are representatives of
benzodiazepines, were synthesized without racemization in
66% and 73% overall yields, respectively. Although some
solid-phase syntheses of 1,4-benzodiazepine-2-ones were
reported,5 our method has the advantages of short reaction
sequences, a variety of building blocks available and no
functional group attached on solid support. In addition, some
other benzodiazepines such as 2,3-benzodiazepine-4-ones17
and pyrrolo[2,1-c][1,4]benzodiazepine-5-ones18 have been
reported to have interesting biological activities such as
AMPA receptor antagonists and antibiotic antitumor agents.
The solid-phase synthesis of these types of compounds will
be also able to be realized utilizing our linker. Synthesis of
the combinatorial library of the benzodiazepine family is in
progress.
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Acknowledgment. This study was financially supported
by a Grant-in-Aid from the Ministry of Education, Culture,
Sports, Science, and Technology of the Japanese Government
(No.18590024) and Sasakawa Scientific Research Grant from
the Japan Science Society.
Supporting Information Available. Synthesis and char-
acterization of all the new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
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CC9001795